Functional fabrics may be produced or modified to provide a specific function. Herein we define a functional fabric as a fabric that is intended to provide a specific function. For example, functional fabrics may be used for water-repellent clothing, permanent press clothing, antimicrobial clothing, and clothing or bednets treated to kill or repel biting arthropods. Functional fabrics may have an additive when they are produced or may have an additive incorporated after they are produced. Functional fabrics can also be treated to produce or enhance the function; such a fabric will be referred to herein as a treated fabric. In a functional fabric the fabric and additive or treatment act together to produce the desired effect.
In use, functional fabrics can lose part or all of their function due to exposure to the environment, laundering, wear and abrasion. In a used functional fabric, it is desirable to determine the functionality remaining, so the user can decide whether to continue to use the fabric or replace it. Herein the term functionality means the level of function. The determination is preferably in the form of a quantitative assay, so the user can determine if the remaining functionality is satisfactory for the situation.
Methods to determine the remaining functionality either directly or indirectly quantify the functionality. In one case, test methods measure the desired function directly; for example, by testing the amount of water that penetrates a water-repellent fabric, or by assaying the effectiveness of an insect-protective garment using live biting insects. Such methods are often expensive, time-consuming, may damage the fabric or garment, and may require resources that are not readily available. In another case, a test method measures the remaining quantity of the additive used to impart the function; for example, in garments treated with permethrin for protection against insects, the permethrin may be extracted using a solvent and quantified by chemical analysis. Such methods can require reagents and analytical instruments that are not readily available in the field, may be expensive and time-consuming, and may damage the fabric or garment. Further, such methods only confirm that a specific additive is present, not that functional fabric performs its function. For example, the insecticide on a treated garment may be present, as determined by chemical analysis, but may not perform the desired function because it is not biologically available, or the fabric may be so worn that it does not form an adequate physical barrier.
Functional fabrics may suffer loss of function during normal use due to fabric wear and due to loss of the additive. Laundering may cause loss of function and is also a source of fabric wear. A practical method to assess the remaining functionality will desirably incorporate some indicator of fabric wear or abrasion.
A desirable method to assay remaining functionality would be simple, inexpensive, rapid, quickly carried out in a laboratory or in the field, require no consumable supplies, and not damage the fabric so that it could (if warranted) continue in use.
Current methods test only a section of the fabric. However, particularly in the case of water-repellent or insect-protective fabrics, if the functionality is not present everywhere in the fabric, then the function is compromised. For example, if a section of a garment is not water-repellent the wearer will get wet in that location. More seriously, if a section of an insect-protective garment does not retain its functionality, then the user could get bitten in that location, potentially acquiring a disease. Therefore, in addition to the desirable properties cited above, it is a further advantage of a simple and rapid assay that it can more readily be applied to multiple locations of the item being tested. For example, in the case of clothing, it could be applied to areas that are known to experience high wear (knees, elbows) and areas that appear to be worn or abraded.
An important example of functional fabrics is certain U.S. military uniforms that are treated with permethrin (a pyrethroid insecticide) to protect soldiers from arthropods that carry diseases, including malaria, dengue fever, and Lyme disease. Efficacy is measured by the bite protection test performed at the USDA Center for Medical, Agricultural, and Veterinary Entomology (CMAVE) in Gainesville, Fla. To pass the test, the uniforms must retain efficacy after 25 standard washes performed according to American Association of Textile Chemists and Colorists (AATCC) Test Method 135-2004. However, it has been found that uniforms worn in the field lose activity faster than predicted by the number of wash cycles they have been through. As shown in Bernier (Page 22 in Ulrich R. Bernier, “Mosquito Bite Protection of Factory-Level Permethrin-Treated United States Military Combat Uniforms” 59th Annual Meeting of the Entomological Society of America, Nov. 13-16, 2011; incorporated by reference herein), test data on treated uniforms worn in Iraq shows that the level of bite protection does not correlate with the number of washings of the uniforms. Furthermore, some uniforms showed negative bite protection; in those cases, the bite protection afforded by the treated, field-worn uniform was less than that provided by a new, untreated uniform. The interpretation is that those uniforms were thinner due to field wear, likely both lacking permethrin and being thinner, thus allowing the insects to bite through the fabric. A treated uniform may need to retain both an effective amount of permethrin that is accessible to insects landing on the surface, as well as sufficient fabric thickness and structural integrity to provide a physical barrier.
Wear and abrasion are examples of mechanisms by which functional fabrics and treated fabrics may lose functionality. Further, as wear breaks down the uniform fibers, water can more easily penetrate and insects (including mosquitoes) can more easily bite the wearer through the fabric.
Existing methods to assess remaining functionality in treated fabrics either measure the property directly, or use a destructive surface sampling and chemical analysis to quantify the amount of remaining additive responsible for the function.
Examples of direct methods include: tests of water repellency, for example AATCC Method 22-2010, which is described as “applicable to any textile fabric, which may or may not have been given a water-repellent finish”. It measures the resistance of fabrics to wetting by water. It is especially suitable for measuring the water-repellent efficacy of finishes applied to fabrics.” In this test, “water sprayed against the taut surface of a test specimen under controlled conditions produces a wetted pattern whose size depends on the relative repellency of the fabric”. Evaluation is accomplished by “comparing the wetted pattern with pictures on a standard chart”. Similarly for permanent press fabrics, AATCC Test Method 124-1996 describes standard conditions to assess the “Appearance of Fabrics After Repeated Home Laundering,” using standard conditions for laundering.
In the case of fabrics treated for protection from arthropod bites, the World Health Organization (WHO) cone test is a standard that requires exposure to live, host-seeking mosquitoes (WHO 2006). WHO cone tests were originally designed to evaluate the toxicity of insecticide-treated bednets against malaria mosquitoes. They are also suited to investigate the toxicity of other impregnated (textile) surfaces. The bite protection test performed by the Center for Medical, Agricultural, and Veterinary Entomology (CMAVE) also uses live mosquitoes (Bernier 2011). This test is used by the U.S. Army to assess the initial performance of their permethrin-treated uniforms.
Examples of destructive tests include: several methods to assess the remaining functionality of garments treated to protect against arthropod bites by sampling, followed by chemical analysis.
Kaur et al. (2013) describe sampling followed by colorimetric assay for pyrethroid insecticides on treated bednets and other items. This technology was developed by the Innovative Vector Control Consortium (IVCC) and has led to a product called the Insecticide Quantification Kit. Similarly, Green and co-workers at the Centers for Disease Control and Prevention (CDC) have described sampling procedures and colorimetric analysis for pyrethroids on surfaces, including fabrics (Green et al. 2009, 2013). The above methods rest on chemical analysis, which can be very specific to the additive (for example, a pyrethroid containing a cyano group); other additives may not respond to the analytical method, requiring development of new chemistry.
Ahn et al (2014) discloses novel chemistry for analysis of certain types of pyrethroids, further emphasizing the challenge of chemical analysis.
Fractal analysis has been used for a wide range of practical applications, such as in the medical field, including fractal analysis of cell images to distinguish between normal and cancerous cells; Bauer (1998) and Sokolov (2014).
These references contain at least one of the following limitations in regard to evaluating functional fabrics: inability to assay functionality non-destructively; requires reagents, solvents or specialized equipment that are not readily available; requires direct measurement of the function, for example live insect tests; is expensive, is inconvenient; or does not utilize fractal analysis to assay functionality.
There remains a need in the art for simple, non-destructive test to assay the functionality of used functional fabrics.